Power ramping for rach

ABSTRACT

The present invention relates to a method for requesting access to a resource, said method comprising a secondary station transmitting an access preamble to a primary station, upon failure of correct transmission of the access preamble, the secondary station starting a first power ramping scheme for transmitting the access preamble, upon failure of correct transmission of the access preamble with the first power ramping scheme, the secondary station transmitting a second access preamble with a second power ramping scheme being different from the first power ramping scheme.

FIELD OF THE INVENTION

This invention relates to a method of gaining access to resources of achannel, like a RACH, and to a secondary station carrying out the methodof the invention, like a mobile station in a mobile telecommunicationsystem. More specifically, the invention relates to a configurableshortened power ramping procedure.

BACKGROUND OF THE INVENTION

In UMTS WCDMA there is a mechanism defined for Random Accesstransmission in the uplink. The UE (User Equipment or mobile station)transmits a randomly-selected preamble signal characterised by thefollowing parameters:

-   -   Signature sequence (i.e. bit sequence),    -   Scrambling code,    -   Sub-channel (i.e. timing of the access slot in the frame).

If the base station receives the signature, it acknowledges it on theAcquisition Indicator Channel (AICH) with a corresponding signature. Ifthe UE receives a positive acknowledgement it transmits a message parton a RACH data channel.

If the UE does not receive an acknowledgement after transmitting thepreamble signal, it can retransmit with a higher power using a procedureknown as power ramping, usually comprising increasing power inpredetermined steps.

If the UE still does not receive an acknowledgement after a number ofsteps (or receives a negative acknowledgement indicated by an invertedsignature on the AICH) it can make another attempt following a furtherrandom selection of the above parameters.

If two or more UEs select the same signature and scrambling code in thesame access slot, a collision occurs, and one or more of the UEs willhave to restart the access procedure. The probability of collisiondepends on the number of Signature sequences, Scrambling codes, Accessslots from which the UEs make their selection tor the randomly-selectedpreamble signal.

According to known procedures, if a UE receives a NACK or a collisionoccurs, it has to start the power ramping procedure again from thebeginning with the next-selected preamble. Typically the initialtransmit power is quite low, usually derived from an open-loop powerestimate, and repeating the ramping procedure can result in significantdelay for the UE to access the network.

SUMMARY OF THE INVENTION

It is an object of the present invention to propose a method enabling aquicker random access scheme.

It is another object of the invention to propose a method of randomaccess which is more configurable.

In accordance with a first aspect of the invention, a method is proposedfor requesting access to a resource, said method comprising a secondarystation transmitting an access preamble to a primary station, uponfailure of correct transmission of the access preamble, the secondarystation starting a first power ramping scheme for transmitting theaccess preamble, upon failure of correct transmission of the accesspreamble with the first power ramping scheme, the secondary stationtransmitting a. second access preamble with a second power rampingscheme being different than the first power ramping scheme.

In accordance to a second aspect of the invention, it is proposed asecondary station comprising requesting means for requesting access to aresource, said requesting means comprising means for transmitting anaccess preamble to a primary station, said requesting means beingarranged for, upon failure of correct transmission of the accesspreamble, starting a first power ramping scheme for transmitting theaccess preamble, and for, upon failure of correct transmission of theaccess preamble with the first power ramping scheme, transmitting asecond access preamble with a second power ramping scheme beingdifferent from the first power ramping scheme.

According to the present invention, if one or both of a collision occursor the UE receives a. NACK, the UE reselects a new preamble signal(combination of signature and/or scrambling code and/or access slot) andtransmits it using a simplified preamble transmission procedure. Thishas the advantage of reducing access delay.

These and other aspects of the invention will be apparent from and willbe elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will now be described in more detail, by way ofexample, with reference to the accompanying drawing, wherein:

FIG. 1 is a block diagram of a system in which is implemented theinvention,

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a system of communication 300 asdepicted on FIG. 1, comprising a primary station 100, like a basestation, and at least one secondary station 200 like a mobile station.

The radio system 300 may comprise a plurality of the primary stations100 and/or a plurality of secondary stations 200. The primary station100 comprises a transmitter means 110 and a receiving means 120. Anoutput of the transmitter means 110 and an input of the receiving means120 are coupled to an antenna 130 or an antenna array comprising aplurality of antennas, by a coupling means 140, which may be for examplea circulator or a changeover switch. Coupled to the transmitter means110 and receiving means 120 is a control means 150, which may be forexample a processor. The secondary station 200 comprises a transmittermeans 210 and a receiving means 220. An output of the transmitter means210 and an input of the receiving means 220 are coupled to an antenna230 or an antenna array comprising a plurality of antennas, by acoupling means 240, which may be for example a circulator or achangeover switch. Coupled to the transmitter means 210 and receivingmeans 220 is a control means 250, which may be for example a processor.Transmission from the primary radio station 100 to the secondary station200 takes place on a downlink channel 160 and transmission from thesecondary radio station 200 to the first radio station 100 takes placeon an uplink channel 260,

Some preferred embodiments of the invention are as follows:

The simplified preamble transmission procedure may comprise one or moreof:

-   -   using a larger power step between preambles    -   starting with a higher power    -   starting with the same power as the last transmission of the        previous preamble.

The use or non-use of the simplified preamble transmission procedure maybe dependent on one or more of the following:

-   -   the particular preamble selected the first time    -   the type of RACH access (for example Release 99 RACH or E-RACH)    -   one or more characteristics of the data or transmissions to        follow the RACH access.

In a typical embodiment, the invention is applied in UMTS WCDMA.

A detailed embodiment is described below.

In R1-074976, “Enhanced Uplink for CELL EACH”, Philips, available athttp://www.3gpp.org/ftp/tsg_ran/WG1_RL1/TSGR1_(—)51/Docs/R1-074976.zip,we discussed a number of issues related to the use of RACH preambles forthe initial phase of starting Enhanced Uplink in Cell_FACH. In thispaper we focus on the E-DCH resource allocation phase.

Note that in this paper, when we refer to “E-DCH Resource” we mean acombination of an UL scrambling code, E-RNTI, F-DPCH code andtime-offset, E-RGCH/E-HICH code and signatures, and E-AGCH code.

E-DCH Resource Allocation Methods Overview of Possibilities

In general, two extreme possibilities may be identified:

-   -   Each E-RACH preamble is directly associated with one E-DCH        resource. The E-DCH resource is effectively selected randomly by        the UE, and no additional resource allocation signalling is        transmitted by the NodeB when acknowledging the preamble. If the        E-DCH resource corresponding to the preamble selected by the UE        is already in use, the NodeB responds to the preamble with a        NACK on the AICH.    -   There is no predetermined association between E-RACH preambles        and E-DCH resources. The allocation of E-DCH resources is        carried out by the eNodeB and signalled in response to an E-RACH        preamble.

Intermediate cases are also possible, where an association existsbetween each E-RACH preamble and a set of E-DCH resources. When the UEselects a preamble it therefore also selects a corresponding set ofE-DCH resources, and the particular E-DCH resource within that set isselected by the NodeB and signalled in response to the preamble.

As discussed in R1-074976, in all cases the total set of E-DCH resourcesavailable should be broadcast, so that any signalling by the NodeB cansimply comprise an index.

E-DCH Resource Allocation Signalling

The main factor governing which of the above methods should be useddepends on how many signalling bits can be transmitted when the NodeBresponds to an E-RACH preamble.

If no bits are available (i.e. the NodeB simply sends a conventionalAICH response with no extension), then option (1) (each E-RACH preambledirectly associated with one E-DCH resource) should be used. However, inour view, the set-up delay associated with option (1) is likely to betoo long. This is because the collision probability becomes limited bythe number of E-DCH resources available, not the number of E-RACHpreambles available. Whenever a UE happens to select a preamblecorresponding to an E-DCH resource which is already in use, it has tostart again with random preamble selection.

Therefore possibilities for reducing the delay of option (1) might beconsidered, for example:

-   -   use the reserved part at the end of the AICH to broadcast the        indices of the E-DCH resources which are not yet in use, so that        the UE can select a corresponding E-RACH preamble, and/or    -   shorten the power ramping phase. For example by omitting the        power-ramping phase of the RACH for a subsequent access attempt        if the UE's first preamble transmission of the previous attempt        is NACK'ed. (If the UE has already established a power level at        which its first preamble was acknowledged, it introduces        unnecessary delay if the UE has to start with a lower power        level when selecting a different preamble. The process could be        speeded up by allowing the UE to use the same power level as for        the previous NACK'ed preamble.)

However, in practice it seems entirely possible to transmit a fewadditional signalling bits together with the AICH response. Possiblemethods for this include:

-   -   using certain existing AICH signatures, or groups of        simultaneously-transmitted signatures, to allocate E-DCH        resources, as proposed in R1-074303, “Resource assignment for        E-DCH access in CELL_FACH state” Nokia Corporation, Nokia        Siemens Networks, available at        http://www.3gpp.org/ftp/tsg_ran/WG1_RL1/TSGR1_(—)50b/Docs/R1-074303.zip    -   extending the number of available AICH signatures, and using        them for signalling, as also proposed in R1-074303.    -   signalling using the reserved part at the end of the AICH.

Method (1) used on its own without extending the information-carryingability of the AICH reduces the number of signatures available foraccess requests and therefore results in an increased collisionprobability. Therefore we do not prefer such a solution.

The number of bits available with each of methods (2) and (3) needsfurther evaluation. In principle, 16 additional signatures are availablewith method (2), although this would generate some additionalinterference to existing R99 AICH responses which needs to be evaluated.The number of bits available with method (3) is more limited (8 bitswith SF256), but does not cause interference to existing AICH responses.

The amount of interference could be reduced in a default resource isindicated by sending only an ACK on the AICH, and in case this resourceis not available, another may be indicated by sending additionalsignalling bits.

Both of these methods 2) and 3) (or even a combination of the two)should be evaluated further.

If the E-DCH resource allocation is indicated entirely by the choice ofE-RACH preamble then methods for reducing the delay might be considered,for example:

use the reserved part at the end of the AICH to broadcast the indices ofthe E-DCH resources which are not yet in use, so that the UE can selecta corresponding E-RACH preamble, shorten the power ramping phase.

If additional signalling bits are used to indicate the E-DCH resourceallocation, we propose to transmit a few additional signalling bitstogether with the AICH response.

Possible methods for this include:

-   -   extending the number of available AICH signatures, and using        them for signalling, as also proposed in R1-074303.    -   signalling using the reserved part at the end of the AICH

In the present specification and claims the word “a” or “an” precedingan element does not exclude the presence of a plurality of suchelements. Further, the word “comprising” does not exclude the presenceof other elements or steps than those listed.

The inclusion of reference signs in parentheses in the claims isintended to aid understanding and is not intended to be limiting.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art. Such modifications may involveother features which are already known in the art of radio communicationand which may be used instead of or in addition to features alreadydescribed herein.

1. A method for requesting access to a resource, said method comprisinga secondary station transmitting an access preamble to a primarystation, upon failure of correct transmission of the access preamble,the secondary station starting a first power ramping scheme fortransmitting the access preamble, upon failure of correct transmissionof the access preamble with the first power ramping scheme, thesecondary station transmitting a second access preamble with a secondpower ramping scheme being different from the first power rampingscheme,
 2. The method of claim 1, wherein the first power ramping schemecomprises transmitting the first access preamble successively with atransmission power increased by a first power step, wherein the secondpower ramping scheme comprises transmitting the second access preamblesuccessively with a transmission power increased by a second power steplarger than the first power step.
 3. The method of claim 1, wherein thefirst power ramping scheme comprises stalling from a first power level,and wherein the second power ramping scheme comprises starting from asecond power level being higher than the first power level.
 4. Themethod of claim 3, wherein the second power level equals the power levelof the last transmission according to the first ramping power scheme, 5.The method of claim 1, wherein the use or non-use of the second powerramping scheme depends on at least one factor of the following: asignaled instruction; the first access preamble; the type of accessrequest; the type of resource for which access is requested; one or morecharacteristics of the data or transmissions to follow access request.6. The method of claim 5, wherein at least one factor determining theuse or non-use of the second power ramping scheme is signaled to thesecondary station by the primary station.
 7. A secondary stationcomprising requesting means for requesting access to a resource, saidrequesting means comprising means for transmitting an access preamble toa primary station, said requesting means being arranged for, uponfailure of correct transmission of the access preamble, starting a firstpower ramping scheme for transmitting the access preamble, and for, uponfailure of correct transmission of the access preamble with the firstpower ramping scheme, transmitting a second access preamble with asecond power ramping scheme being different from the first power rampingscheme.
 8. A secondary station for use in a network of one or moreprimary and multiple secondary stations, the secondary stationcomprising: one or more antennas, a coupler connected to the one or moreantennas to, at alternate times, rout transmissions to the antennas androut receptions from the antennas, a transmitter with an input and anoutput, the output being connected to the coupler to transmit thetransmissions, a receiver with an output and an input, the input beingconnected to the coupler to receive the receptions, and a processorconnected to the input of the transmitter and the output of the receiverto control the transmitter and the receiver for communicating with theprimary station, and wherein the secondary station transmits an accesspreamble to a primary station, and upon failure of correct transmissionof the access preamble, the secondary station starts a first powerramping scheme for transmitting the access preamble, and upon failure ofcorrect transmission of the access preamble with the first power rampingscheme, starts a second power ramping scheme, being different from thefirst power ramping scheme, and transmits a second access preamble.